Fix array access in a null pointer dereference

FcFontSetFont() accesses fs->fonts in that macro though, there was no error checks
if it is null or not.
As a result, there was a code path that it could be a null.
Even though this is unlikely to see in usual use, it might be intentionally created
in a cache.
So if fs->fonts is a null, we should consider a cache is invalid.